1. Field of the Invention
This invention relates to head-up display systems.
2. Description of Related Art
More particularly the invention relates to head-up display systems of the kind (hereinafter referred to as "head-up display systems of the kind specified") comprising: an object surface; a holographic combiner which has been constructed using two coherent sources respectively located adjacent to entrance and exit pupils of the combiner so as to deviate an axial ray coming from the object surface through an off-axis angle into an exit pupil of the system and which has a focal surface with axial coma and axial astigmatism; and an optical sub-system along the path of said axial ray between the object surface and the combiner which at least partially compensates said axial coma and said axial astigmatism. Such a system is described in US Pat. No. 3940204.
It will be understood that in such a system the optical sub-system forms at the focal surface of the holographic combiner an aberrated intermediate image of the object surface; and that, by using aberrated wavefronts in the hologram construction, the holographic combiner is formed with aberrations balancing those of the intermediate image, the aberrations thus being at least partially compensatory so that a virtual image of the object surface at infinity is seen by an observer looking through the combiner from the exit pupil, which virtual image is of useful quality.
When such a system is used as a pilot's display system in an aircraft cockpit, to accommodate the system to the spatial constraints of the aircraft cockpit, the optical path between the object surface and the holographic combiner is folded by a reflector means whereby rays from the object surface are redirected at the exit of the optical sub-system to the combiner for reflection thereby to the exit pupil.
In known such arrangements, as are described in the aforementioned US Pat. No. 3940204, the reflector means comprises a prism which is positioned beneath the combiner, in line with the pilot ejection plane of the cockpit, so that there is no room available forward of the ejection plane to accommodate additional display arrangements such as a secondary head-down display arrangement.